As is known in the art, frequency bands within the radio frequency spectrum can be used for multiple purposes, such as radio communications and radar. In some cases, it may be desirable to collocate the antennas of multiple systems that are operating within the same frequency band. Typically, each of these antennas will be designed to transmit and/or receive radio frequency electromagnetic energy with high gain over a limited angular range, or main lobe. However, such antennas also transmit and receive electromagnetic energy, at lower gain, at other angles outside of the main lobe. Thus, radio frequency interference can occur between multiple systems even if each antenna is positioned outside of the main lobes of the other collocated antenna.
FIG. 1 shows a prior art antenna installation 100 having a first antenna 110 and a second antenna 120 collocated within radome 140. In the illustrated installation, the first antenna 110 is a so-called LINK-16 communications antenna operating in the L-band at frequencies from 960 MHz to 1215 MHz. As is well known in the art, Link-16 provides a U.S. government-backed system for transmitting broadband data across a variety of air, sea and ground-based platforms. The second antenna 120 is a combination of an L-band radar and an IFF (Identification Friend or Foe) antenna operating at frequencies of 1030 MHz and 1090 MHz. The IFF antenna is located at the upper portion of the antenna combination 120.
In the illustrative installation, the first antenna 110 has a torroidal main lobe that covers 360 degrees in azimuth and roughly 20 degrees in elevation centered on horizontal. The second antenna 120 has a narrow fan beam that is scanned by means of a mount 130 that rotates antenna 120 about the vertical axis.
While each of the first and second antennas 110, 120 are positioned outside of the main lobe of the other antenna, radio frequency interference can still occur due to stray or side lobe radiation between the antennas, as shown schematically by dashed lines 150. Of particular concern in this example is radiation from the communications antenna 110 degrading the performance of the radar/IFF system, which has high sensitivity in order to receive radar returns from distant objects.